This invention relates generally to turbine engines and, more particularly, to apparatus and methods for cooling an airfoil of a turbine engine.
A turbine engine typically includes a core engine having, in serial flow relationship, a high pressure compressor which compresses an airflow entering the core engine, a combustor in which a mixture of fuel and compressed air is burned to generate a propulsive gas flow, and a high pressure turbine which is rotated by the propulsive gas flow. The high pressure turbine may be connected to the high pressure compressor by a shaft so that the high pressure turbine drives the high pressure compressor. Additional compressors and turbines (e.g., a low pressure compressor and a low pressure turbine) may be positioned in serial flow relationship with the core engine. As used herein, the term "turbine" includes, without limitation, high pressure turbines and low pressure turbines
Cooling of engine components, such as components of the high pressure turbine, is necessary due to thermal stress limitations of materials used in construction of such components. Typically, cooling air is extracted air from an outlet of the compressor and the cooling air is used to cool, for example, turbine airfoils. The cooling air, after cooling the turbine airfoils, re-enters the gas path downstream of the combustor.
Known turbine airfoils include cooling circuits through which cooling air flows for cooling the airfoil. More particularly, internal cavities within the airfoil define flow paths for directing the cooling air. Such cavities may define, for example, a serpentine shaped path having multiple passes (e.g., three or five passes). In general, a five-pass cooling circuit has an increased cooling effectiveness as compared to a three-pass cooling circuit.
Fabricating an airfoil having a five-pass cooling circuit, however, is more complex and expensive than fabricating, for example, an airfoil having a three-pass cooling circuit. More particularly, airfoils typically are fabricated using a die cast process. A casting core for a five-pass airfoil cooling circuit typically has a more complex shape, a smaller cavity dimensions, and is more fragile than a casting core for a three-pass airfoil cooling circuit.
In addition, during operation, the flow variation through a five-pass airfoil cooling circuit is greater than the flow variation through a three-pass airfoil cooling circuit due to higher airflow velocity, and an associated higher pressure loss, through the five-pass cooling circuit. Controlling air flow through a five-pass airfoil cooling circuit therefore is more difficult than controlling air flow through a three-pass cooling circuit. Until now, increased fabrication costs and decreased air flow control of a five-pass cooling circuit were trade-offs for increased cooling effectiveness.
Accordingly, it would be desirable to provide an airfoil having a cooling circuit which is less complex and less expensive to fabricate than a five-pass cooling circuit airfoil, yet has an increased cooling effectiveness as compared to known three pass cooling circuits.